That's a good point, I am not a lawyer so I can't really comment. But perhaps there is a material difference between saying you developed a disease that could have been caused by smoking (as lung cancer occurs in non-smokers), and saying you were definitely harmed by smoking.

Other avenues could be:

- Smoking is also associated with other cancers, such as esophageal cancer and bladder cancer. Generally the warnings don't mention this however. By examining the mutation patterns in an esophageal cancer, you could relate it more conclusively to smoking, and could therefore claim you were not warned about that specific risk.

- Tissue samples from lung cancers diagnosed 20 or 30 years ago are sitting in archival storage. You could sequence the whole genome or exome of these tumours for about $1000 - $2000. Statute of limitations aside, sequencing these tumours could reveal the tobacco signature as a basis for a claim from a time when it was less clear or less public what the risks were.

- Passive smokers could have a case if their cancer shows a tobacco signature.